Problem with getting Sample Size

As Software Development company with 225 customers wishes to
assess the effect on sals of a price change on their consultancy services. From past records their mean annual revenue per customer s known to be 350 euro with a standard deviation of 50 euro.
The company wishes to estimate the new average revenue after the price change to within 10euro of the true value with 95% confidence. How many Customers should be sampled?

Ok here i baiscally want to get the Sample size. I think i should use this formula